基于自发瑞利-布里渊散射的氮气体黏滞系数的测量

体黏滞系数是从微观角度认识气体分子黏滞性的重要参数,传统的兆赫兹声频范围的声波吸收方法无法直接应用于声波弛豫效应在千兆赫兹范围的高频领域,而瑞利-布里渊散射则能实现对声波弛豫效应在千兆赫兹的气体体黏滞系数的测量.本文测量了532 nm激光激发的常温下压强分别为1-9 bar的氮气的自发瑞利-布里渊散射光谱,利用已知温度和压强的理论模型对测量光谱进行了比较,获得了准确的散射角.利用该散射角并结合χ~2值最小原理反演得到不同压强(4—9 bar)下氮气的平均体黏滞系数为(1.46±0.14)×10~(-5)kg·m~(-1)·s~(-1),该结果与文献中利用自发瑞利-布里渊散射获得的结果和理论计算结果相近,但与相干瑞利-布里渊散射的测量结果相差明显.利用该平均体黏滞系数对氮气在不同压强下的温度进行了反演,得到各压强下的温度与实际温度的绝对误差小于2.50 K,反演温度的平均值与实际温度误差小于0.15 K,该结果证明了实验测量得到的氮气的体黏滞系数具有较高的准确性,同时也说明利用瑞利-布里渊散射反演气体参数具有较高的准确性和可靠性.

Measurement of bulk viscosity of nitrogen based on spontaneous Rayleigh-Brillouin scattering

Bulk viscosity is an important parameter to understand gas viscosity in micro perspective. The traditional ultrasound absorbtion method with acoustic frequencies in a megahertz range cannot be directly applied to high frequencies field, where acoustic waves are in the gigahertz domain. However, gas bulk viscosity at high frequency can be measured by spontaneous Rayleigh-Brillouin scattering (SRBS) and coherent Rayleigh-Brillouin scattering (CRBS). Recent researches show that the bulk viscosity of nitrogen measured by CRBS at a wavelength of 532 nm is obviously different from the values from SRBS in the near-ultraviolet region. In order to obtain accurate bulk viscosity of nitrogen at the wavelength of 532 nm, the SRBS spectra of nitrogen excited by a 532 nm laser are measured in a pressure range from 1 bar to 9 bar at the constant room temperature. The measured SRBS spectrum at the pressure of 7 bar is compared with the theoretical spectrum to obtain optimal scattering angle by using the principle of minimum value of χ². The theoretical spectrum is calculated by convolving the Tenti S6 model with the instrument transmission function of measurement system. Given that the effect of pressure on the bulk viscosity is negligible, the bulk viscosity value (1.46±0.14)×10^-5 kg·m^-1-1 of nitrogen at a temperature of 299 K is acquired by averaging the values of bulk viscosity under different pressures (4-9 bar), each value is obtained by comparing the measured spectra at different pressures with the theoretical spectra by using the optimal scattering angle and the principle of minimum value of χ². The values of bulk viscosity of nitrogen over the pressure of 1-3 bar are not considered because of its big deviation compared with the values under higher pressures (4-9 bar). The results show that the average value of bulk viscosity obtained in our experiment is close to that from the theoretical calculation and SRBS experiments reported in the literature but different obviously from the bulk viscosity obtained by CRBS. In order to testify the bulk viscosity of nitrogen measured in our experiment, it is used to retrieve temperature of nitrogen under pressure ranging from 1 bar to 9 bar. The results show that the absolute error between the retrieved temperature and the reference temperature under different pressures are all below 2.50 K and the difference between the average temperature and the reference temperature is less than 0.15 K. This demonstrates that the measured bulk viscosity of nitrogen in our experiment is accurate and reliable for the gas parameters retrieved by SRBS.